May 12, 2011, 7:56 p.m.
posted by demx
When a message from one participant results in one or more messages being sent by the receiving participant, those resulting messages are said to be nested within the triggering message, as shown in Figure.
Two nested messages are invoked when an initial message is received
In Figure, participant1 sends initialMessage(..) to participant2. When participant2 receives initialMessage(..), participant2 becomes active and sends two nested messages to participant3. You can have any number of nested messages inside a triggering message and any number of levels of nested messages on a sequence diagram.